I have to confess that I haven't read this entire thread but, having lived this once and, having a sister that I'm not overly fond of myself, I thought I would weigh in here.
R2D2, unlike your situation, as much as I despise my sister (and I really do mean despise her) I knew from the time that I was engaged that she was going to be invited to my wedding.
As much as my wedding was a day for me and my late wife, it was also a day for my parents and her parents to take some pride in their accomplishments. My wife and I knew that there would be guests there that A) we didn't know and B) we didn't want, but due to family obligations, would have to be invited anyway. On top of this, my wife's parents had full designs on inviting 300 people for their side alone.
In an effort to curtail some of this madness, my wife and I made the decision that each of our parents would be able to invite 100 guests and my wife and I would invite 50 between the two of us. Although the parents' guest list was much longer, it was communicated that parents would be responsible for inviting ALL family members (siblings, spouses and guests of siblings, cousins, aunts, uncles and grandpa Sam on his day pass from Kingston Pen). All of those guests would be the responsibility of our respective parents and it was up to them to make sure that the important ones are included.
I would suspect that part of the reason that your daughter does not want your son at the wedding is because there is a limitted number of places and that he just may not be a priority. I would also suspect that your son, no matter how big an ass he can be, would, more than likely have the decorum to put aside his youthful angst for a day. Yes he may look like a freak and maybe your daughter won't want him in the wedding pics (aside from some family shots) but my guess is that if you introduce the "if you're an ass I'm going to rip every piercing out of your head one by one" threat in a constructive way, the little bit of trust that you give him to not be a dick may pay off in spades.
So, in a nutshell, I'd say put him on your guest list. If your daughter protests explain to her how much it would hurt you for your brother not to be there and that, as much as this is her day, as a father of three children, this normally should rank in the top ten happiest days of your life and you would hate to ruin it by not having your only son there.
As for you not attending the wedding? Under no circumstances should you not attend this wedding. Tension between siblings is one thing but by not attending the wedding you would be taking sides in a situation where there seems to be plenty of fault. In fact, as father, you would be abdicating a MAJOR responsibility to your child and ruining one of the most important days of her life.
It may seem a little bit of a cliche, but with the holidays drawing near you may wish to leverage this time of year to try to try to mitigate this wedding situation. Don't try to fool yourself in thinking you will fix their relationship. I promise you that you will not. Odds are that if there is that much animosity between these two than the day when you and their mother are dead and gone will be the day that they no longer communicate in any way ever. That having been said, even nations at war during WWI were able to put aside their differences for a day each Dec. 25th to play a game of soccer between the trenches. Certainly, your children can put their differences aside for a wedding. If not to make themselves a little bit more happy then at least to make you a little happier. It's the least that they can do for their dad.
No matter which way you go, best of luck and I hope everything works out for you and your entire family.
